The present invention relates to the production of polyethylene resins which are suitable for forming film having low MVTR (moisture vapor transmission rate). The present invention especially relates to such processes for producing polyethylene resin using a heterogeneous catalyst comprising chromium and titanium on a solid support.
The polymerization of ethylene to produce polyethylene using a catalyst comprising chromium or chromium and titanium on solid support, such as silica, has been known for several decades. An early reference in this area is U.S. Pat. No. 2,825,721 to Hogan and Banks. A second group of catalysts for ethylene polymerization are Ziegler catalysts, comprising titanium halide and aluminum alkyl, also have been extensively used to produce polyethylene. A third group of catalysts for ethylene polymerization are catalysts comprising molybdenum, as described, for example, in U.S. Pat. No. 2,692,257. However, molybdenum catalysts have not been extensively used for ethylene polymerization. More recently, a fourth group of catalysts, metallocenes, comprising a cyclopentadienyl substituted transition metal, also have come into use.
Of these four groups of catalysts, the present invention is concerned with ethylene polymerization processes using a catalyst comprising chromium on a solid support, such as a silica.
Numerous references have been published concerning use of chromium-silica catalysts for polyethylene production, including references wherein the chromium component is impregnated onto a support such as silica and references where chromium and silica are formed as a cogel. Example references wherein the catalyst is formed by an impregnation procedure include U.S. Pat. No. 2,825,721. Examples of forming the catalyst by cogelation include U.S. Pat. No. 5,115,053 (cogel of chromium and silica) and U.S. Pat. No. 5,183,792 (tergel of chromium and titanium and silica).
For the chromium-silica catalysts which include titanium as a component, the titanium is generally introduced or added to the chromium-silica catalyst in one step. Thus, for instance, U.S. Pat. No. 4,294,724 states that it is known that titanium affects the polymerization activity of silica supported chromium catalysts in a way that is of special importance in slurry polymerization. However, when titanium is co-precipitated with the silica, it produces a hydrogel which does not have sufficient strength to resist serious collapse of the pores during simple drying, such as spray drying. Accordingly, in order to take full advantage of the improvement which can be imparted to the melt index capability through the use of titanium in accordance with the prior art, the titanium had to be co-precipitated with the silica and the resulting hydrogel (cogel) dried by a more expensive azeotrope distillation or washing with a liquid oxygen-containing water soluble organic compound.
References such as McDaniel""s xe2x80x9cSupported Chromium Catalysts for Ethylene Polymerizationxe2x80x9d, Advances in Catalysis (1985) pages 47-98, teach that the amount of titanium in the catalyst should not be high, as high titanium levels lead to sintering and reduce surface area. Thus, McDaniel states at page 78: xe2x80x9cThat titania increases the termination rate also can be seen in FIG. 14. Here the melt index, which reflects the termination rate of some co-precipitated sample, is plotted against the titania concentration. At 650xc2x0 C. and 760xc2x0 C. calcining temperatures, the melt index increases with titania content, but at 870xc2x0 C., a peak in melt index is obtained, followed by a sharp drop. This is due to sintering, which can be considered as the earliest stages of melting. Sintering destroys the surface area and porosity of the catalyst. Although Cr/silica itself does not sinter at 870xc2x0 C., the added titania does promote sintering, as impurities often lower the melting point of solids. Both activity and MI potential are diminished by sintering, and the more titania added, the more easily the catalyst sinters.xe2x80x9d
Turning now to MVTR, as pointed out in WO 96/19527 to Davis, one of the most common polyolefin polymers valued for its low moisture or water vapor transmission rate (MVTR) is high density polyethylene (HDPE). Generally, HDPE""s are those which have densities at or above about 0.940 g/cc. Generally, the higher the density, the better a resin""s MVTR for a given package thickness. The Davis reference refers to use of a metallocene catalyst, citing Davis"" co-pending patent application U.S. Ser. No. 08/093,901 which discloses metallocene catalyst systems that can be used to produce polymers having not only excellent strength, sealing, and optical properties, but having superior water vapor transmission rates. The polymers are disclosed to have use in the packaging industry. A film is disclosed having at least one layer having a density less than about 0.935 g/cm3, a Mw/Mn less than about 3, a CDBI greater than about 80%. The layer includes a resin having a density about 0.90 g/cm3 and an MVTR of less than about 2.25 gxc2x7mil/100 in2/day. Thus, Davis is directed to improved (lower) MVTR through use of a resin having a relatively narrow MWD. Davis points out that in certain embodiments of his invention, the resin produced has a density in the range of from about 0.935 to about 0.965 g/cm3, a Mw/Mn less than about 3, and an article made using the resin has a water vapor transmission rate less than 0.54 gxc2x7mil/100 in2/day, preferably less than 0.4 gxc2x7mil/100 in2/day. Also, in Plastics Technology, August 1999, in an article by J. Krohn et al. titled xe2x80x9cKeep It Dry, Optimize Moisture Barrier in PE Filmsxe2x80x9d, at pages 60-61, the authors state xe2x80x9cThus, structure 3 excelled in barrier because it was the only one to have a skin layer of higher MI resin with narrower MWD, both of which contribute inherently to better barrier.xe2x80x9d
According to the present invention, a process is provided for polymerizing ethylene to form a polyethylene homopolymer having an MWD greater than 4 and suitable for forming a film having an MVTR less than 0.3 gxc2x7mil/100 in2/day, which process comprises contacting the ethylene under slurry or gas phase reaction conditions with a chromium-silica cogel catalyst containing titanium, wherein the catalyst contains titanium added in at least two steps: (1) titanium added as part of a first cogel formation, and (2) titanium added in a post-titanation step after the first cogel catalyst is formed and dried.
Preferably, the polyethylene polymer produced in accordance with the present invention has an MVTR below 0.25 gxc2x7mil/100 in2/day, most preferably less than 0.2 gxc2x7mil/100 in2/day.
Also, preferably the polyethylene polymer produced in accordance with the process of the present invention has an MWD greater than 4, more preferably greater than 5, and most preferably greater than 6. The MWD may be as high as 10; more preferably the high range of MWD is about 8.
The term xe2x80x9ccogelxe2x80x9d as used in connection with the present invention is used to embrace cogellation of two components, such as the chromium and silica, as well as cogellation of three (more precisely, a xe2x80x9ctergelxe2x80x9d) components, such as chromium-titanium and silica.
According to a preferred embodiment of the present invention, the amount of titanium in the first cogel formation is sufficient to produce a first cogel containing between 1 wt. % and 5 wt. % titanium, more preferably 1.5 wt. % to 4 wt. % titanium, and most preferably 2 wt. % to 3 wt. % titanium, based on the dried first cogel. The amount of titanium added in the post-titanation step in accordance with the present invention preferably is sufficient so that the post-titanated catalyst contains between 5 wt. % and 15 wt. % titanium, more preferably between 6 wt. % and 12 wt. % titanium, and most preferably between 7.0 wt. % and 9.5 wt. %, based on the calcined catalyst.
For the catalyst used in the process of the present invention, preferably the amount of titanium added by post titanation and preferably also the calcination conditions for the catalyst used in the process of the present invention are controlled to produce a polymer with highly effective microstructure for MVTR performance, especially controlled with respect to the balance of MWD and long chain branching.
Preferred final calcination temperature for the catalyst used in the present invention is 1000xc2x0 F. to 1600xc2x0 F., more preferably 1100xc2x0 F. to 1500xc2x0 F., and most preferably 1200xc2x0 F. to 1400xc2x0 F.
Preferred heat up and calcination conditions for the catalyst used in the present invention include, after drying of the post-titanated catalyst, a heat up period in a nitrogen rich atmosphere, from a start temperature below 300xc2x0 F., preferably below 200xc2x0 F., up to an intermediate temperature between 500xc2x0 F. and 800xc2x0 F. Preferably, the catalyst is brought up to the intermediate temperature level gradually, using a ramp up rate of between 50xc2x0 F. and 250xc2x0 F. per hour. Preferably, the catalyst is held at the intermediate temperature for one to four hours. Then the catalyst preferably is calcined, in air, preferably at a final temperature between 1000xc2x0 F. and 1600xc2x0 F., more preferably between 1200xc2x0 F. and 1500xc2x0 F., and most preferably 1100xc2x0 F. to 1400xc2x0 F. Preferably, the catalyst is held at this final temperature for 2 to 48 hours, more preferably 4 to 24 hours. Preferably, the catalyst is brought up to the final temperature gradually, using a heat up rate between 50xc2x0 F. and 250xc2x0 F. per hour, and preferably carrying out the ramp up, between the intermediate temperature and the final calcination temperature, in an atmosphere of air.
Preferably, the resin produced with the post titanated catalyst in the process of the present invention, usually in the form of polymer flake, is extruded and pelletized under conditions to produce minimal or no changes to the polymer microstructure, including minimal or no crosslinking during this step of the process. Preferably, changes to the polymer microstructure, such as crosslinking of the polymer, is minimized or avoided by selection of extrusion configurations and conditions and also by using polymer stabilizers. The extrusion configuration and conditions can include screw speed, screw design, extrusion temperatures, and output rate. Stabilizers can include a combination of phenolic and phosphite type antioxidants.
Among other factors, the present invention is based on our finding that use of a post-titanated catalyst, that is, a catalyst prepared in two titanation steps in accordance with the present invention, produces a resin of relatively broad MWD, yet having, when formed into film, a surprisingly low MVTR. Further, we have found that the resin made using a post-titanated catalyst according to the present invention has surprisingly advantageous processing properties, particularly when the resin is processed to form film.
Resins produced under the present invention process generally require approximately 15% less extruder pressure during the film extrusion process compared to existing HDPE MVTR resins of equivalent melt index. In addition, resins produced under the present invention usually demonstrate equivalent melt strength to resins produced using standard Crxe2x80x94Ti-Silica catalyst without post-titanation and approximately 10% better melt strength to competitive HDPE MVTR resins of similar melt index. The combination of lower pressure required during extrusion and higher melt strength allow the processor of the resin manufactured under the present invention to run these resins at higher processing outputs than present HDPE MVTR resins of comparable melt index values.